为什么half an hour later 中an不变成a

a half hour还是half an hour? 是half hour还是half an hour? half an hour or a half hour? half an hour later half an hour造句 Half an hour. half an hour later she woke up.还有这half an hour为什么等于thirty minutes. 为什么half an hour later 中an不变成a half hour =half an hour half hour half an hour 的意思有和区别? before后可以接half an hour He went to Hangzhou before half an hour .before 后可以接”half an hour ”.为什么 Half an hour ___,my mum came back.A.after B.late C.behind D.later she was half an hour late for class this morning because there was （) with her bike 英语翻译Mr Black usually comes back from work half and an hour late. The train I was traveling on was already half an hour late. Sam ( )here half an hour ago括号中应填什么?为什么? I spend half an hour ___to school.为什么填getting? why did you come so late?the bus was crowed.we were kept for half an hour.为什么用were kept? an hour and half还是an hour and a half